This invention relates to a transfer loaded on a four wheel drive vehicle and capable of changing the drive mode between two wheel drive and four wheel drive and a control apparatus for providing such a change between two wheel drive and four wheel drive.
This type of transfer is generally designed such that the rotational force of the rear wheel output shaft is transmitted to the front wheel output shaft via a clutch and a power transmission mechanism whereby a selection is made between four wheel drive and two wheel drive by connecting and disconnecting the clutch. One typical tranfer is briefly described. Referring to FIG. 15, there is illustrated in schematic view the apparatus disclosed in Japanese Patent Application Kokai No. 56-39352 which is incorporated herein by reference. A region defined by dot-and-dash lines in FIG. 15 designates an overall automatic transmission/transfer 1 including main and auxiliary transmissions 2 and 3 both having a planetary gear mechanism, a clutch, and a brake as main elements. The main transmission 2 is connected to an engine 5 through a torque converter 4. A front wheel output shaft 7 extends parallel to a rear wheel output shaft 6 which extends from the auxiliary transmission 3 as its output shaft. The rear and front wheel output shafts 6 and 7 are connected via a clutch 8 and a power transmission mechanism 9. An intermediate shaft 10 is coaxially mounted on the rear wheel output shaft 6 and connected to the rear wheel output shaft 6 through the clutch 8. A chain 13 is extended over sprockets 11 and 12 secured to the intermediate shaft 10 and the front wheel output shaft 7, respectively. Two wheel drive is achieved with the clutch 8 disconnected because the rotational force imparted to the rear wheel output shaft 6 from the engine 5 is no longer transmitted to the front wheel output shaft 7. Four wheel drive is achieved by connecting the clutch 8. A selection can thus be made between two wheel drive and four wheel drive by connecting and disconnecting the clutch 8.
Four wheel drive vehicles were traditionally most often used as transport in mountainous, snowy, and desert areas. The recent trend of diversified civil demands is also true in the vehicle industry. There is a need for small size passenger cars to be of four wheel drive type for recreation and other purposes. In an automatic transmission/transfer of the type as shown in FIG. 15, however, the transfer comprised of the two/four wheel drive changeover clutch 8 and the power transmission mechanism 9 is aligned with the main and auxiliary transmissions 2 and 3, and the entire distance from the engine 5 to the end of the rear wheel output shaft 6 is thus considerably long. It is then difficult to load vehicles having a relatively short wheel base such as small-size passenger cars with such an automatic transmission/transfer. If loaded, the passenger compartment must be reduced in volume, detracting from the available space and comfort.
In the arrangement shown in FIG. 15, since the front wheel output shaft 7 is connected to a front axle via a propeller shaft and a differential gear mechanism (not shown), connection of the clutch 8 allows the power to be transmitted to the front wheels. Differently stated, the power transmission mechanism 9 comprised of the chain 13 and the intermediate shaft 10 is interlocked with the front wheel output shaft 7 and the propeller shaft with respect to rotational motion. Then, a so-called intermediate mechanism including the power transmission mechanism 9 and the front wheel output shaft 7 is rotated by the power transmitted from the rear wheel output shaft 6 and even by the power transmitted from the front wheels. Consequently, when the clutch 8 is disconnected to provide for two wheel drive, the rotational force of the rear wheel output shaft 6 is not directly transmitted to the power transmission mechanism 9 and the front wheel output shaft 7, but the rotation of the front wheels during running causes the front wheel output shaft 7 and the power transmission mechanism 9 to unnecessarily rotate, leading to a power loss, and hence, problems of low fuel consumption and the like.
In general, automatic transmission/transfer combined systems are hydraulically controlled. When the engine is stopped, control is lost because of the loss of any available hydraulic pressure. In order to stop the rotation of the rear wheels in parking state, a parking equipment is added wherein a parking lock pawl engages with a parking gear attached to the outside of a member integrally combined with the rear wheel output shaft, for example, a clutch drum. In the four wheel drive vehicle automatic transmission/transfer of the arrangement shown in FIG. 15, however, the two/four wheel drive changeover clutch 8 is generally a wet clutch operable by a hydraulic oil pressure as used in the main and auxiliary transmissions 2 and 3. With this arrangement, when the car is stopped in the four wheel drive mode and the engine is stopped to provide the parking state, the rear wheels are locked by the aforementioned parking equipment. The front wheels, however, are left for free rotation because the clutch 8 receives no hydraulic oil pressure due to the stopping of the engine and is thus maintained disconnected. As a result, the car is kept standstill only by the rear wheels. Differently stated, a force required to keep the car standstill is supported by the two rear wheels as soon as the engine is stopped.
A further problem involved in the prior art four wheel drive vehicle transmission/transfer shown in FIG. 15 is a power loss due to co-rotation that the power transmission mechanism 9 is rotated together with the front wheels even in the two wheel drive mode. This also adversely affects fuel consumption.
Such an inconvenient problem may be eliminated by disposing an additional clutch between the front wheel output shaft 7 and the power transmission mechanism 9. The additional clutch interrupts the rotational force transmitted from the front wheels in the two wheel drive mode, thereby preventing the unnecessary rotation of the power transmission mechanism 9. The provision of the two clutches is effective in that the power transmission mechanism 9 is maintained stationary by releasing both the aforementioned two/four wheel drive changeover clutch 8 and the additional clutch in the two wheel drive mode to interrupt power transmission. The power transmission mechanism 9 is not rotated unless both the clutches are connected to provide the four wheel drive. In the two wheel drive mode, the rear wheel output shaft 6 is rotated by the power from the engine 5 and the front wheel output shaft 7 is rotated upon receipt of the rotational force from the front wheels. If a change is made from the two wheel drive to the four wheel drive during run, the power transmission mechanism 9 receives rotational forces from both the rear and front wheel output shafts 6 and 7. At this point of changeover to the four wheel drive mode, the power transmission mechanism 9 quickly starts rotating from the stationary state and must transmit a great power at the same time so that the engagement characteristics of the respective clutches are unstable in transient state, probably resulting in the reduced life of the clutches and the power transmission mechanism.
The aforementioned clutches generally employ hydraulic clutches in order to enable connection during run and enable interlocking with the automatic transmission mechanism. When a certain hydraulic pressure is applied to both the clutches to provide the four wheel drive, not only the transient engagement characteristics of the respective clutches are unstable as mentioned above, but there can occur a reduction in the hydraulic pressure, inviting the relaxation of clutch connection and adversely affecting other clutches in case of an automatic transmission/transfer. It is thus very difficul to optimize control parameters.
The provision of two clutches for the selection between two wheel drive and four wheel drive thus requires a special control operation.